The invention relates to a method for quench correction of samples emitting ionizing radiation in a liquid scintillation counter, possibly with the use of an external standard.
A liquid scintillation counter detects ionizing radiation, such as, for example, beta radiation, from an unknown sample introduced in a scintillator solution comprising a solvent and a fluorescent substance. The fluorescent substance then changes from its excited state to its normal state while emitting photons. The higher the energy of the emitting isotope, the more molecules of the fluorescent substance are excited and the stronger is the intensity of the emitted flash of light of the respective excitation or scintillation, of the fluorescent material. The electrical signal emitted by a measuring transducer, e.g. two coincidences connected photomultipliers, is generally proportional to this light intensity and thus proportional to the energy of the nuclear radiation actuating the excitation. In order to distinguish between nuclear radiations, or decay events of different energy, the electrical pulses emitted by the measuring transducer are discriminated according to pulse height. The spectra of various pulse height ranges can thus be associated with various electron radiation emitting nuclides, which may possibly be contained in the respective sample and differ in their beta spectra.
However, losses occur during the detection of nuclear radiation events.
Controllable losses whose specific parameters can be kept constant in a controlled manner during the entire measurement, e.g. from among a plurality of losses, those due to incomplete light collection, will not be discussed here in detail.
Practically the only uncontrollable loss is the loss from so-called quenching. Depending on whether the light emission itself is impaired in the course of converting the energy produced by the nuclear radiation or whether it is the subsequent light transmission into the photoelectric transducers that is impaired, a distinction is made between chemical quenching and color quenching. In practice, there exist neither qualitatively nor quantitatively predictable cases of a mixture of chemical and color quenching.
The counting yield ZA is understood to mean the quotient of the actual counts per minute, cpm, counted after pulse height discrimination and the real disintegrations per minute, dpm. If the counting yield is known, it is possible to determine the rate of disintegration, which is what is of ultimate interest, from the measured pulse rate according to the equation ##EQU1## This can be done by either summarily determining the counting yield without distinguishing between chemical quenching and color quenching, which, however, considerably reduces accuracy; or, as will be explained in detail below with the aid of a novel method, both quenching factors can be considered separately.
There exist a number of prior art methods for determining the counting yield ZA. Pointed out in particular should be the internal standard method wherein, after measuring the counting rate, a known quantity of calibration radioactivity is added to the sample and the sample is counted again; the sample channels ratio method, wherein the counting yield is determined via the pulse rate ratio between two pulse height ranges of the sample spectrum and is brought into a relationship with calibration measurements at increasing degrees of quench, the quenching effect shifting the pulse spectrum toward lower energies; and the external standard channels ratio method wherein, in a modification of the sample channels ratio, the channels ratio is changed by irradiation of the sample with an external standard, e.g. a gamma radiator such as .sup.137 Cs, generating a Compton spectrum which is more or less superposed on the sample spectrum, the Compton spectrum being subject to a shift analogous to that of the sample spectrum.
Regarding the details of the prior art on which the species of the present invention is based, the terminology employed and the mechanisms serving as its basis as well as the prior art generic technique for determining quench corrected counting yields, reference is made particularly to German Offenlegungsschrift No. 2,521,904 by the same applicant, particularly the introduction to the specification up to page 13, and to U.S. Pat. No. 4,075,480, particularly the introduction to the specification up to column 4, line 55. Applicable modern prior art is also disclosed in U.S. Pat. Nos. 4,029,401 and 4,060,728 as well as German Offenlegungsschriften [applications published without examination] Nos. 1,623,050, 2,725,750 and 2,726,840. Each of the prior art methods for determining the quench corrected counting yield have certain inherent drawbacks. Reference is further made to the latter part of the detailed description of this specification which defines various technical terms that form the basis of the invention herein.
The method according to U.S. Pat. No. 4,075,480 uses the position of an inflection point in a pulse height spectrum as a measure for the degree of quench. However, the position of this point is greatly dependent on statistical fluctuations as well as on very slight differential nonlinearities of the entire amplifier arrangement, particularly of a logarithmic amplifier. Moreover, often an inflection point cannot be observed at all in the pulse height spectra actually under observation. In some sections, the descent seems to be defined by a straight line so that after an inflection point a search circuit may arrive at results which are due to random events.
In the conventional external standard channels ratio method, ESCR, it may happen that the spectra shift from fixed, preselected channels and thus any quench correction will become considerably less accurate or, in the borderline case, not possible at all. Moreover, as in other prior art methods for determining the quench corrected counting yield, the sensitivities of the measurement are often unsatisfactory.